Control of carbon balance in a silicon smelting furnace by monitoring calcium

Abstract: A method is described for controlling the carbon balance of a silicon smelting furnace by maintaining the concentration of calcium within the silicon formed in the furnace within a range of 0.4 to 2.0 weight percent. The calcium yield of the furnace is a rapid and sensitive indicator of the carbon balance of the furnace. More specifically, a calcium yield of less than 80 weight percent indicates a negative carbon balance in the furnace; a calcium yield of greater than 90 weight percent indicates an excess carbon balance in the furnace; and a calcium yield within a range of 80 weight percent to 90 weight percent indicates a furnace in carbon balance. A preferred embodiment of the present invention uses calcium yield in conjunction with a harmonic volts parameter and a electrode consumption measurement to further control carbon balance of the furnace.

Description: The present invention introduces a method for controlling the carbon balance of a silicon smelting furnace. We have found that when the concentration of calcium within the silicon formed in the furnace is maintained within a range of 0.4 to 2.0 weight percent, the calcium yield of the furnace is a rapid and sensitive indicator of the carbon balance of the furnace. More specifically, a calcium yield of less than 80 weight percent indicates a negative carbon balance in the furnace; a calcium yield of greater than 90 weight percent indicates an excess carbon balance in the furnace; and a calcium yield within a range of 80 weight percent to 90 weight percent indicates a furnace in carbon balance. In a preferred embodiment, we use calcium yield in conjunction with a harmonic volts parameter andan electrode consumption measurement to control carbon balance of the furnace. Elemental silicon is produced by the carbothermic reduction of silicon dioxide (SiO 2 ) according to the overall reaction: SiO 2 + 2C → Si + 2CO Those skilled in the art recognize that this reaction proceeds through a number of intermediate reactions involving the production and reaction of silicon monoxide (SiO) and silicon carbide (SiC). Important intermediate reactions are summarized as: The reaction of silicon dioxide with carbon to form silicon carbide, SiO 2 + 3C → SiC + 2CO the reaction of silicon monoxide with carbon to form silicon carbide, SiO + 2C → SiC + CO and the reaction of silicon with carbon to form silicon carbide, Si + C → SiC As equation (1) suggests, a silicon smelting furnace is in carbon balance when two moles of carbon are added to the furnace per mole of silicon dioxide. This condition is described as 100 percent carbon theory. However, due to process inefficiencies, operation of a silicon smelting furnace does not proceed exclusively according to reaction (1). If there is insufficient or unreactive carbon present in bulk or locally, reaction (3) is affected and a portion of the silicon monoxide will exit the charge bed in the offgas. This situation can occur due to raw materials selection, bed design and unbalanced stoichiometry in the bed. The loss of silicon monoxide from the furnace results in reduced recovery of elemental silicon. In addition, in the case of insufficient carbon, increased consumption of carbon electrodes used in the furnace can occur. In extreme cases of carbon deficiency, carbon used as furnace lining may also be consumed. Conversely, if too much carbon is present in the furnace, in bulk or locally, reactions (2), (3) and (4) can collectively cause silicon carbide accumulation and reduced silicon production. The product silicon carbide will accumulate and fill the furnace thereby causing the electrode to raise out of the proper operating position. In addition, when excess silicon dioxide is added to react with the accumulated silicon carbide, the additional silicon monoxide formed can result in increased electrode consumption and loss of process yield of elemental silicon. Therefore, it is important for efficient furnace operation that the furnace be kept in carbon balance. However, as a result of the described inefficiencies, carbon balance in a silicon smelting furnace cannot be maintained by merely adding carbon and silicon dioxide to the furnace in a two to one molar ratio, based on carbon theory alone. Therefore, it is an objective of the present invention to provide a method wherein the carbon balance of a silicon smelting furnace can be determined within a fairly short time period (four to eight hours) and effective adjustments can be made to the carbon balance as needed. The benefits provided by our method include reduced electrode consumption, higher process yields and less furnace shutdowns. Dosaj et al., U. S. Patent No. 5,174,982, describe a method for assessing the carbon balance of a silicon smelting furnace by measuring the amount of carbon monoxide evolved in offgas exiting the furnace. Halvorsen, U. S. Patent No. 4,539,194, claims adding one to ten percent calcium to silicon to facilitate subsequent purification steps. Dosaj et al., U. S. Patent No. 4,798,659, teach an improved process for the preparation of silicon by the reduction of silicon dioxide with a solid carbonaceous reducing agent. The improvement involves feeding calcium compounds into the reaction zone of a silicon furnace while controlling and maintaining a desired calcium level in the reaction zone of the silicon furnace. The calcium compounds may be fed to the silicon furnace as a constituent of either the silicon dioxide or solid carbonaceous reducing agent feeds, as a separate feed or as a combination of two or more of these feeds. The cited art does not teach or suggest the use of calcium yield of a silicon smelting furnace as an indicator of carbon balance within the furnace, thus providing control of the carbon balance and enhanced furnace performance. The present invention provides a method for controlling the carbon balance of a silicon smelting furnace comprising: (A) providing to the furnace a mixture comprising silicon dioxide, a carbon source and a calcium source, (B) applying energy to the furnace to effect carbothermic reduction of silicon dioxide to silicon, (C) controlling calcium concentration within the silicon within a range of 0.4 to 2.0 weight percent, (D) recovering silicon from the furnace, (E) determining calcium yield in the recovered silicon to determine carbon balance of the furnace, where a calcium yield of less than 80 weight percent indicates a negative carbon balance in the furnace; a calcium yield of greater than 90 weight percent indicates an excess carbon balance in the furnace and a calcium yield within a range of 80 weight percent to 90 weight percent indicates a furnace in carbon balance, and (F) adjusting carbon balance of the furnace based on the calcium yield. The present invention is practised by controlling the carbon balance of a silicon smelting furnace. A mixture comprising silicon dioxide, a carbon source and a calcium source is provided to the furnace. Energy is then provided to the furnace to effect the carbothermic reduction of the silicon dioxide. The energy provided to the furnace is by any known means, for example, a submerged-arc electrode or a transferred-arc plasma torch. The electricity used as the energy source can be direct current or single and multiphase alternating currents. A preferred means for supplying energy to the furnace is by one or more submerged-arc carbon or graphite electrodes connected to an alternating current source. The components comprising the mixture of this invention may be added to the furnace separately and in combination. Other additives may also be charged to the furnace, for example, phosphorous as described in U. S. Patent No. 4,898,960 and up to two weight percent copper. The silicon dioxide added to the furnace may be quartz in its many naturally occurring forms; and fused and fumed silica, precipitated silica and silica flour in their many forms. The physical form of the silicon dioxide can be as powder, granule, chip, lump, pellet, flake and briquette. Preferred is when the silicon dioxide is quartz. The source of carbon added to the furnace can be charcoal, coal, coke, wood chips or mixtures thereof. The form of the carbon can be granule, chip, lump, pellet, flake and briquette. The calcium can be provided to the furnace from several sources. The calcium may be added as a constituent of the silicon dioxide or carbon source provided to the furnace or by a combination of these two feed materials. Silicon dioxide varies significantly in calcium content, depending upon the source of the silicon dioxide. As an example, it has been found that the calcium content of naturally occurring quartz used in the manufacture of silicon can vary in a range from 0.003 to 5 weight percent. Correspondingly, the calcium content of the carbon source can vary significantly depending upon the source. An example of a carbon source with high calcium content is oak wood chips which can have a calcium content of between 0.2 to 0.4 weight percent. An example of a carbon source with low calcium content is fir wood chips with a calcium content of about 0.02 weight percent. Therefore, those skilled in the art will recognize that calcium content of the silicon formed in the present process can be controlled at least partially by proper selection of feed materials. A calcium compound may be added as a source of calcium to the furnace either alone or in combination with other feed components. The calcium compound can be selected from calcium oxide, calcium carbonate and calcium fluoride. The calcium compound may be fed to the furnace as a solid material. The form of the solid calcium compound can be powder, granule, chip, pellet, flakes, particles, lumps and briquettes. The calcium compound may also be fed as a water solution or slurry. This water solution or slurry can be fed by any conventional means for feeding liquids such as pumps, gravity feed or gas-pressure feed. The concentration of calcium within the silicon recovered from the furnace must be controlled within a range of 0.4 to 2.0 weight percent in the practice of this invention. Preferred is when the concentration of calcium within the silicon recovered from the furnace is controlled within a range of 0.5 to 1.0 weight percent. Controlling the calcium level within these ranges is effected by analyses of the starting raw materials and of the silicon recovered from the furnace and altering the feeds to maintain the desired concentration within the furnace. Analyses for calcium content of the feeds and recovered silicon can be by standard techniques known in the art such as spectrographic means. Control of the calcium concentration within the described ranges can be effected by selection of appropriate feed materials and by the addition of calcium compounds as previously described. Recovery of silicon from the furnace can be effected by standard means, such as periodic or continuous tapping into a ladle or mold. If desired, the silicon may be refined by standard oxidative and slag-forming methods. For purposes of the present invention, the term "calcium yield" (C Y ) is defined as the weight percent of calcium in silicon recovered (C R ) from the furnace divided by the weight percent of calcium added (C A ) to the furnace as feed from all sources, the quotient multiplied by 100, i.e., C Y =C R /C A X 100. The inventors believe that the ability of the calcium yield of the furnace to predict the carbon balance of the furnace is predicated by the following two reactions occurring in the furnace: 3SiC + CaO = CaC 2 + 3Si + CO and CaC 2 + 2SiO 2 + 2C= Ca + 2Si + 4CO When there is a negative carbon balance in the furnace then a dicalcium silicate slag (2CaO·SiO 2 ) forms reducing yield of calcium. When there is an excess of carbon in the furnace then excess silicon carbide is formed which can push the electrode up in the furnace and effect heat and energy distribution within the furnace. When the furnace is in carbon balance, the reactions described by equations (5) and (6) predominate and calcium is recovered in the silicon within the desired range of 80 to 90 weight percent. The carbon balance of the furnace is adjusted based on the calcium yield. The carbon balance is adjusted by altering the ratio of silicon dioxide to carbon source provided to the furnace. The amount of this alteration will depend upon the particular furnace and its operating conditions and can be easily ascertained by those familiar with the operation of such furnaces. In a preferred embodiment of the present invention, the harmonic volts and the electrode consumption are monitored in conjunction with calcium yield to control the carbon balance of the furnace. The harmonic volts can serve as a leading indicator of carbon imbalance within the furnace while the electrode consumption can serve as a lagging indicator of carbon imbalance within the furnace. Skreien, U. S. Patent No. 3,909,242, discloses a method for determining operating conditions in electrical furnaces. Skreien teaches that the measured electrical parameter, harmonic volts, is the distortion of the electrode holder-to-furnace bottom voltage or hearth voltage, as compared to the supplied grid voltage. We have found that the harmonic volts parameter is a leading indicator of carbon imbalance within a silicon smelting furnace when used in conjunction with our calcium yield measurements as previously described herein. Specifically, when harmonic volts exceed a certain maximum, which may vary by furnace, more carbon is added if calcium yield is below 80 weight percent. Conversely, when harmonic volts are lower than a specified minimum, which may vary by furnace, carbon addition to the furnace is reduced if calcium yield is greater than 90 weight percent. In addition, as a lagging indicator, a furnace in carbon balance will maintain an electrode consumption below 100 kg per tonne of silicon produced. A furnace with a negative carbon balance will have greater electrode consumption. The following example is provided to illustrate that calcium yield can be used as a means to control the carbon balance of the furnace. Example A commercial 18 megawatt furnace equipped with three 1143 mm carbon electrodes connected to a three-phase AC 25 MVA transformer was operated for a two-month period without using calcium yield as an indicator of carbon balance in the furnace. During this period the silicon recovery was 76 percent, energy utilization was 13.5 mWh per tonne of silicon produced and electrode consumption was 110 kg per tonne of silicon produced. The furnace was then run using calcium yield as an indicator of carbon balance within the furnace. The calcium level within the silicon recovered was maintained within a range of 0.5 weight percent to 0.75 weight percent by adding limestone to the furnace as a feed, when required. The carbon balance of the furnace was periodically adjusted to keep the yield of calcium from the furnace within a range of about 80 weight percent to 90 weight percent. Over a four month period of operating the furnace in this mode, silicon recovery was 78.8 percent, energy utilization was 12.8 mWh per tonne of silicon produced and electrode consumption was 93.5 kg per tonne of silicon produced.

Claims: A method for controlling the carbon balance of a silicon smelting furnace comprising: (A) providing to the furnace a mixture comprising silicon dioxide, a carbon source and a calcium source, (B) applying energy to the furnace to effect carbothermic reduction of silicon dioxide to silicon, (C) controlling calcium concentration within the silicon within a range of 0.4 to 2.0 weight percent, (D) recovering silicon from the furnace, (E) determining calcium yield in the recovered silicon to determine carbon balance of the furnace, where a calcium yield of less than 80 weight percent indicates a negative carbon balance in the furnace, a calcium yield of greater than 90 weight percent indicates an excess carbon balance in the furnace and a calcium yield within a range of 80 weight percent to 90 weight percent indicates a furnace in carbon balance, and (F) adjusting carbon balance of the furnace based on a calcium yield of less than 80 weight percent or of greater than 90 weight percent. The method of claim 1 further comprising the measurement of harmonic volts as a leading indicator of carbon balance within the furnace and the measurement of electrode consumption as a lagging indicator of carbon balance within the furnace. The method of claim 1 where energy is provided to the furnace by one or more submerged-arc carbon or graphite electrodes connected to an alternating current source. The method of claim 1 where the silicon dioxide is quartz. The method of claim 1 where the source of calcium comprises the silicon dioxide, the carbon source and a calcium compound. The method of claim 1 where the calcium concentration within the silicon is controlled within a range of 0.5 to 1.0 weight percent.

Inventors: SCHOONOVER DAVID EARLE, SILVER RONALD FOUQUET, YOUNG MARTIN DAVID, SCHOONOVER, DAVID EARLE, SILVER, RONALD FOUQUET, YOUNG, MARTIN DAVID
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C01B33
IPC codes: C22B4, C01B33